In connection with the recovery of hydrocarbons from the earth, wellbores are generally drilled using a variety of different methods and equipment. According to one common method, a roller cone bit or fixed cutter bit is rotated against the subsurface formation to form the wellbore. The bit is rotated in the wellbore through the rotation of a drill string attached to the bit and/or by the rotary force imparted to the bit by a subsurface fluid motor powered by the flow of drilling fluid through the drill string.
A problem associated with normal rotary drilling of this type, particularly when a fixed bit configuration is used, is that a stall condition can result once a certain torque load threshold at the bit is reached, in which case the bit may drag or stop rotating completely. When a bit stalls, typically, the attached drill string continues to turn, which can result in damage to the drill string and/or bottom hole assembly. Even if the operating torque applied through the string eventually succeeds in breaking the bit free of the formation, i.e., overcoming the torque load on the bit resulting in a stall, the sudden release of the bit can cause it to rotate faster than the drill string, resulting in a condition referred to as stick-slip. Stick-slip can cause problems in the operation of the drilling assembly and in the formation of the well bore.
Conventional techniques to reduce the incidence of damage to the motor, drill string, bottom hole assembly or wellbore occurring as a result of high torque loads include increasing shaft sizes and utilizing enhanced or alternative materials. However, while technology has continued to develop more powerful driving systems, there are limits to the size and materials that can reasonably be used for the various components. For example, annulus dimensions limit the radial size of the components, while cost considerations may limit material choice.
Thus it would be desirable to provide a drilling system that can operate at higher torques without increasing the likelihood that a peak torque threshold will be crossed, while at the same time minimizing component dimensions and material costs.